Lead halide perovskite semiconductors have been extensively studied for a variety of optoelectronic applications, including photovoltaics (PVs), photodetectors, and LEDs. There has been considerable interest in these materials due to their tunable optical properties, large absorption coefficients, excellent mobilities, long excited state lifetimes, low cost, and simple solution-based deposition techniques by which most are commonly formed. Since the first report of these materials as light absorbers in single-junction photovoltaics in 2009, power conversion efficiencies exceeding 20% have been realized and rationalized owing to their excellent photophysical properties.
Intermediate band (IB) PVs are a class of multi-junction devices that have the theoretical ability to surpass the Shockley-Quessier of single p-n junction cells. IB PVs require an IB material sandwiched between two conventional (n- and p-type) semiconductors, which serve as selective contacts to the conduction band (CB) and valence band (VB), respectively. These PVs are designed to retain the high output voltages characteristic of large band gap semiconductors while harvesting significantly more of the available solar spectrum by absorbing pairs of sub-band gap photons to produce additional high-energy carriers. Only a handful of examples of materials that exhibit the properties necessary for IB PV operation have been reported to date. GaAs is the most successful parent material to date owing to its excellent photophysical properties, to which perovskite halides have been compared, as well as the ability to form quantum wells with smaller gap. While the GaAs-quantum wells operate as IB PVs, the sub-parent-bandgap contribution to the photocurrent is meager as it is limited by the achievable density of the structurally strained quantum wells. The bandgap of GaAs is also not ideal, as the optimal total band gap of an IB material has been calculated to be approximately 2.0 eV, which is split by a mid-gap level into two sub-band gaps of approximately 0.7 eV and 1.3 eV (FIG. 1). In this arrangement, the maximum photovoltage is the difference between the quasi-Fermi levels (i.e. the electrochemical potentials of the electrons) in the CB and VB of the n- and p-side electrodes (2.0 eV).
Lead halide perovskites may be prime candidates for this application due to their outstanding photophysical properties, suspected structural tolerance for metal substitution, and tunable band gap. Compared to conventional inorganic binary semiconductors like GaAs (band gap=1.4 eV), a large compositional space has been identified for APbX3 perovskites (A=cations such as Cs+, methyl ammonium [MA], or formamidinium [FA] and X═Cl, Br, and/or I). While some elements have been substituted for Pb in the APbX3 perovskite structure, those have been in the same group IV, Sn and Ge, as well as related (periodic table near neighbors) Bi and Hg. These alternatives have garnered interest as a compromise in order to avoid the toxicity of Pb but at the expense of stabilities and device performances as compared to their Pb-containing analogues. Recently, Bi has also been utilized in conjunction with Ag to form the double perovskite Cs2AgIBiIIIBr6, which exhibits an encouraging photoluminescence lifetime, but only a single gap, and only in single crystal form.